Cardiac resynchronization therapy (CRT) is one therapy used to treat heart failure (HF) patients. During CRT, pacing pulses are delivered to one or more heart chambers to restore synchrony of the heart chambers. Guidelines for selecting patients for CRT have been established, e.g. New York Heart Association (NYHA) classification III to IV, left ventricular ejection fraction (LVEF) 35% or less, and a wide QRS complex of 120 ms or more. Despite these guidelines, it may be possible that not all patients benefit from CRT. Some patients, referred to as “responders,” present clinical improvement, e.g. improvement in NYHA class or other clinical measures, and may present improvement in a cardiac function measurement, e.g. decrease in left ventricular end systolic volume (LVESV), in response to CRT therapy. Other patients selected for CRT, referred to as “non-responders”, may present no improvement, clinically or hemodynamically. Clinical end points for evaluating the efficacy of CRT are still evolving. Because of the varied response to CRT between patients meeting current therapy selection guidelines, patient selection for CRT and subsequent follow-up and evaluation continues to be challenging to clinicians.
Once a patient is selected for a pacing therapy, optimizing timing of pacing parameters is important in achieving therapeutic benefit of the pacing therapy, or at least preventing unintentional deleterious hemodynamic effects of the pacing therapy. Pacing timing control parameters include the atrial-ventricular (AV) interval used during single chamber ventricular pacing, during dual chamber (atrial and ventricular) pacing, and multi-chamber pacing, and the intra-ventricular (VV) interval used during biventricular or multichamber pacing. The AV interval is a pacing control time interval started upon sensing an atrial event (P-wave) or delivering an atrial pacing pulse. Upon expiration of the AV interval, the pacing device delivers a ventricular pacing pulse if an intrinsic ventricular event (R-wave) is not sensed during the AV interval. Similarly, the VV interval is used to control the timing of a ventricular pacing pulse following a programmed W interval after a paced or sensed R-wave occurring in the other ventricular chamber.
Echocardiography continues to be a “gold standard” for optimizing pacing timing parameters. Echocardiography, however, tends to be a costly and time-consuming procedure which requires specially trained sonographers to perform. Other methods for monitoring hemodynamic performance of the heart typically require relatively more invasive techniques such as cardiac catheterization for measuring left ventricular (LV) pressure, cardiac output or other standard hemodynamic measurements. As such, the frequency that such techniques can be used to determine the best pacing parameters for an individual patient are limited due to time, cost, burden on the patient, and/or known inherent risks associated with invasive methods. It would be desirable for a clinician to know with a relatively high certainty beforehand whether a patient will be responsive to a given therapy and how best to manage the therapy to avoid or minimize costly, time-consuming and invasive procedures.
Other methods proposed for selecting CRT patients and optimizing CRT therapy include measuring the QRS width measured from ECG signals and performing adjustments to cause a narrowing of the QRS width. This technique assumes that a wider QRS width indicates greater ventricular dyssynchrony and a narrowing of the QRS width will be associated with improved ventricular synchrony. In a significant number of patients, however, electrical dyssynchrony and mechanical synchrony do not strongly correlate. As such, patient selection and optimization based on QRS width may have limited utility.
A need remains, therefore, for a medical device system and associated method for predicting a patient hemodynamic response to a therapy and for enabling identification of patients that can potentially benefit from a cardiac therapy. Furthermore, a technique that provides an indication of how a therapy can be tailored to a given patient to maximize patient benefit and minimize unneeded procedures is desirable.